This invention relates generally to noise reduction devices for turbofan engines and more specifically to a device for simultaneously reducing rotor and stator source noise from the fan stage of a turbofan engine.
In the recent efforts to reduce the noise emitted from jet engines considerable attention has been given to the fan stages of modern high bypass ratio turbofan engines. The two primary noise sources in these fan sections are known to be the fan rotor, and fan stators. In the effort to reduce the noise generated by these components, it has been necessary to first attempt to find the relative importance of the rotor and stator sources, and second, to attempt to define and understand the mechanisms by which the noise is generated.
Until recently most experiments were formed to investigate fan stage noise in static tests performed on the ground and the results were contaminated by poor fan inflow which was not representative of actual flight conditions. In such static tests atmospheric disturbances and obstructions in the fan inflow cause unsteady velocities that interact with the rotor and generate noise peculiar to the particular static test arrangement. The information gained from these tests relating to fan and stator noise has been somewhat misleading. For example, on the basis of such tests it has been generally concluded that noise from the rotor is dominating in the area in front of the engine whereas aft of the engine rotor and stator noise are of equal significance. It appeared then from these older tests that in order to reduce noise in the far field in front of the engine, efforts should be concentrated on the more difficult task of reducing fan noise.
In recent tests, however, the inflow to the fan has been controlled to better simulate in flight conditions and the results have indicated that noise from the stator is more important in the forward arc than previously believed. The tests have indicated that when the fan is operating at subsonic relative tip speeds, simulating engine operating conditions during a landing approach, the noise from the stator contributes significantly to the total noise in the forward arc. In fact, these tests have indicated that for relative fan tip speeds up to approximately sonic speed, stator noise is quite significant in the forward arc. At higher tip speeds, rotor blockage effects on forward radiated stator noise is sufficient that the fan noise dominates.
In the effort to reduce noise generated by the rotor, considerable attention has been focused on the region near the rotor tips. It is recognized that this region is a major source of noise because of the high tip speeds, the interaction of the rotor with the inlet boundary layer and secondary flows from the tips. Investigators in the past have attempted to reduce rotor tip noise by removing the inlet boundary layer just upstream of the rotor, reducing inflow distortion, and reducing some of the rotor tip secondary flows. Typical of these efforts are the devices described by C. J. Moore in U.S. Pat. No. 3,730,639 and A. R. Howell in U.S. Pat. No. 3,735,593.
Stator generated noise on the other hand is generally attributed to rotor-induced unsteady velocities interacting with the stator vanes. The major sources of these unsteady velocities are wakes from the rotor blades and secondary flow from the rotor tip and hub regions. Further, stator tone noise is thought to be due to the harmonic content of these unsteady velocities, and the broadband noise due to random variations. Much theoretical and experimental work has been done in analyzing the effect of the rotor wakes on the stators, but relatively little effort has been made in investigating the effect of rotor tip flow on them. A recent study of the subject suggests that secondary flows originating from the tips and roots of the rotor blades may be more significant in the stator noise problem than the rotor wakes. The present invention was made by the inventors in the course of their efforts to apply these recent theories and test results to achieve a reduction in fan stage noise.
By this invention a moderate to high bypass ratio turbofan engine is enclosed in a nacelle having a controllable duct system which has openings around the periphery of the fan casing. When the ducts are open, a small percentage of the flow in the region after the rotor and relatively near the inner nacelle surface is passed through the openings into the duct system and it is exhausted into the atmosphere. This portion of the flow is thus made to bypass the fan stators and has accordingly been referred to as a stator bypass system. When in operation the stator bypass system reduces both direct rotor and rotor/stator interaction noise by (1) unloading the rotor, (2) reducing secondary or viscous flow at the rotor tips, (3) causing a diffusion of the flow between the rotor and stators, and (4) bypassing a substantial portion of the rotor tip flow around the stators.
The term rotor unloading means a reduction in the back pressure downstream of the rotor resulting from an increase in the effective secondary nozzle exit area. Unloading causes an increase in the mass flow through the rotor and a decrease in the rotor pressure ratio. Also, the incidence angle of the rotor blades is decreased and the relative mach number is increased. Unloading of the rotor moves the rotor operating point away from the stall line thus reducing rotor secondary flows and improves rotor stability. In addition, the velocity deficit of rotor wakes is decreased which in turn causes a reduction in the inflow velocity variations at the stator.
The second effect of this invention is to reduce viscous flow in the region of the rotor tips. When the stator bypass system is in operation, the flow field upstream of the bypass duct openings in the region of the rotor tips is accelerated. The amount of acceleration depends upon the flow conditions behind the rotor, the geometry of the duct openings, and the ambient static pressure. Due to this acceleration in the rotor tip region, the effects of viscous flow rotor tip secondary flows, and rotor sensitivity to inflow velocity variations are reduced.
A further effect of the stator bypass system is to cause a diffusion of the flow between the rotor and stators. The principal effect of this diffusion is to reduce the mean flow velocity at the leading edges of the stator blades. Both the axial and rotational components of the mean flow velocity are reduced so that the stator incidence angle remains closer to design condition than would result if the fan were unloaded by simply increasing the secondary nozzle exit area aft of the stators. Other effects of the diffusion include an increase in the "reduced frequency" of the rotor wake/stator interaction and a reduction in the wake velocity deficit.
Another important effect of the stator bypass system is that it diverts substantial portion of the highly turbulent flow from the rotor tips including the tip vortices, into the bypass duct system, thus preventing this flow from striking the stators. Accordingly, the noise that would be generated by the interaction of this turbulent flow with the stator blades is substantially eliminated.
This invention has direct application to current moderate to high bypass ratio turbofan engines and can be incorporated with only modest structural changes in the engines. Incorporating this invention in a turbofan engine effects the engine cycle match characteristics in a manner similar to an increase in the fan exit nozzle area. When in operation, the system will cause an increase in the fan mass flow rate, relative mach number, bypass ratio and static thrust but will result in a decrease in fan pressure ratio, fan nozzle velocity, and cruise thrust. But since it is intended that this invention only be used during approach and take-off, cruise thrust will not be effected. Accordingly, it can be seen that a principal advantage of this invention is that it provides an appreciable reduction in both rotor and stator noise without effecting engine cruising performance.
This invention is applicable to any turbofan engine and will require varying degrees of cycle rematch depending on the design bypass ratio of the engine. The invention should be most suitable for use on engines having a single fan stage and having a spacing of at least two rotor chords between the rotor and the stators. The invention may be applied, however, to multi-stage fans having alternate rows of rotors and stators where the spacing between the rows is at least one rotor chord.